Isolation, characterization and sequential reaction of the shark complement components that react between Cln and the two terminal components, Tl and T2. Relationship between the first component, Cln, and the C4 inactivator, C41. Structural comparison between mammalian Cl and Cln. Elucidation of the C41 precursor system. Isolation and characterization of the C41 inhibitor found in mammalian sera; its uniqueness or relation to the Cl-esterase inhibitor. Further studies regarding the new concept of chemotactic stimulation by immune reactions occurring at the surface of the responding cell. The influence of shark natural antibody against sheep erythrocytes and antibody-complement component complexes on the rosette formation with human peripheral lymphocytes.